1. Field of the Invention
The present invention relates to an optical reproduction method, an optical pickup device, an optical reproduction device for reproducing data in an optical recording medium, and the optical recording medium.
2. Description of the Related Art
In recent and continuing years, optical recording media, such as a CD having recording capacity of 0.65 GB and a DVD having recording capacity of 4.7 GB, are widely used for storing video data, audio data, and digital data in computers. Presently, it is being required to further increase recording density and the recording capacity of the optical recording media.
In order to increase the recording density of such an optical recording medium, it can be attempted to reduce the wavelength of a light beam incident on the optical recording medium, assign the recording data to multiple levels, or include multiple recording layers in the optical recording medium.
Below, these techniques are described in detail.
Reduction of Light Beam Wavelength
In an optical recording device for recording or reading data in an optical recording medium, by increasing the numerical aperture (NA) of an object lens, or decreasing the wavelength of a light source, it is possible to effectively reduce the diameter of a beam spot formed on the optical recording medium after a light beam is condensed by the object lens.
For example, in a CD-type optical recording medium, the numerical aperture (NA) of the object lens may be set to be 0.50, and the wavelength of the light source may be set to be 780 nm; in contrast, in a DVD-type optical recording medium, the numerical aperture (NA) of the object lens may be set to be 0.65, and the wavelength of the light source may be set to be 660 nm.
As described above, it is desired to further increase the recording density and the recording capacity of the optical recording medium, and for this purpose, it is desired to increase the numerical aperture (NA) of the object lens to be greater than 0.65, or reduce the wavelength of the light source to be shorter than 660 nm. It is known that such a large-capacity optical recording medium and optical recording device can be realized by using a light source operating in the blue-light wavelength region to ensure large recording capacity.
For example, this technique is described in ISOM2001 abstract “Next Generation Optical Disc”, Hiroshi Ogawa, pp 6-7 (referred to as “reference 1”, hereinafter).
However, if the numerical aperture (NA) of the object lens is increased too much, or the wavelength of the light source is decreased too much, margins of various kinds of variations in the optical recording medium may decline. For example, coma aberration caused by the tilt of the optical recording medium may increase. If the coma aberration occurs, the beam spot formed on the recording surface of the optical recording medium may be degraded, so that normal recording and reproducing operations cannot be performed.
The coma aberration caused by the tilt of the optical recording medium can be expressed by the following formula (1).W31=(n2−1)/(2n3)×(d×NA3×θ/λ)  (1)
where W31 represents the coma aberration, n represents the refractive index of a transparent substrate of the optical recording medium, d represents the thickness of the transparent substrate, NA represents the numerical aperture (NA) of the object lens, λ represents the wavelength of the light source, and θ represents the tilt of the optical recording medium.
From the formula (1), it is known that the comma aberration (W31) increases when the numerical aperture (NA) of the object lens increases and the wavelength of the light source decreases.
Similarly, spherical aberration caused by differences of transparent substrates of the optical recording medium varies in proportion to NA to the power of 4, and λ to the power of −1; thus the spherical aberration increases when the numerical aperture (NA) of the object lens increases and the wavelength of the light source decreases.
Multilevel Information
To avoid the above problems, a multi-level recording and reproduction method is proposed in which a signal associated with a pit formed on the optical recording medium is assigned multiple levels. Specifically, in this technique, when a laser beam used for reading an optical recording medium is scanning the optical recording medium, reading data from the optical recording medium is realized by determining variations of the amount of the reflected light from the optical recording medium, with the amount of reflected light depending on presence or absence of pits on the optical recording medium.
In contrast, for example, in the method of multi-level recording and reproduction disclosed in Japanese Laid Open Patent Application No. 58-215735 (referred to as “reference 2”, hereinafter), reading data from an optical recording medium is realized by determining variations of the amount of the reflected light from the optical recording medium, with the amount of light depending on a combination of depth and width of each pit on the optical recording medium.
In addition, in the method of multi-level recording and reproduction disclosed in Japanese Laid Open Patent Application No. 7-121881 (referred to as “reference 3”, hereinafter), reading data from an optical recording medium is realized by determining variations of the amount of the reflected light from the optical recording medium, with the amount of light depending on a combination of depth, width, and variation of a position shift of each pit on the optical recording medium.
In addition, Japanese Laid Open Patent
Application No. 2002-157734 (referred to as “reference 4”, hereinafter) discloses a multi-level recording method related to occupation by (presence of) recording marks, in which it is described that when phase pits having depressed and projected shapes are used, optical groove depths of the phase pits are set to be λ/4 so that a gain of a Rf signal becomes a maximum.
Multi-Layer Recording Layer
Further, it is proposed to form an optical recording medium including plural recording layers. For example, Japanese Laid Open Patent Application No. 8-096406 (referred to as “reference 5”, hereinafter) and Japanese Laid Open Patent Application No. 9-054981 (referred to as “reference 6”, hereinafter) disclose such an optical recording medium including multiple recording layers.
In the optical recording medium including multiple recording layers, an interval between two recording layers is important. If this interval is too small, because of so-called “interlayer cross-talk”, the returning light, which is reflected from the optical recording medium, includes not only light reflected from the object recording layer but also a considerable amount of light reflected from other recording layers. Due to this, the signal-to-noise ratio of the reproduced signals may be degraded. On the other hand, if the interval between two recording layers is too large, because of the spherical aberration, signals associated with light reflected from the object recording layer may be degraded.
For these reasons, in the design of a multi recording layer optical recording medium, for example, in a DVD-ROM having two recording layers on one side thereof, the interval between the two recording layers is defined to be 55±15 μm.
As described above, in order to increase the recording capacity of the optical recording medium, attempts can be made to reduce the wavelength of the recording light beam, or assign multiple levels to recording data, or include multiple recording layers in the optical recording medium. Furthermore, it is expected that the recording capacity can be greatly increased by combining the above techniques.
However, the above techniques and their combinations suffer from the following problems.
First, consider the technique of multi-recording layers in an optical recording medium.
For example, when combining the technique of reducing the wavelength of the recording light beam and the technique of forming multiple recording layers, if the wavelength of the recording light beam is shortened, the upper limit of the interval between two recording layers becomes less than the upper limit of the DVD-ROM having two recording layers on one side thereof (70 μm). For this reason, it is difficult to divert the existing manufacturing equipment, and the fabrication cost of the optical recording media increases.
Next, consider the technique of assigning multiple levels to recording data. It is known that the optical recording media employing the techniques of multi-level signal formation disclosed in reference 2 and reference 3, which are recorded with multi-level data including a pit depth and a pit width, are poor in productivity (low yield).
FIG. 18 is a flowchart illustrating a process for fabricating a read only optical recording medium in the related art, which is recorded with two-level signals (a high level and a low level), as signals reproduced from pits formed on a CD-type or a DVD type recording medium.
As illustrated in FIG. 18, the fabricating process includes a laser cutting step (step S1), a developing step (step S2), a stamper formation step (step S3), and a replication step (step S4).
FIG. 19A through FIG. 19D are cross-sectional views of a read only optical recording medium illustrating the fabrication process of the read only optical recording medium in the related art, as shown in FIG. 18.
As illustrated in FIG. 19A, first, a portion of a resist 102, where a pit is to be formed, on a glass substrate 101 is exposed by a light beam 100 until the glass substrate 101 is reached.
Next, as illustrated in FIG. 19B, developing is performed.
Next, as illustrated in FIG. 19C, based on the developed prototype recording medium, a master stamper is fabricated.
Next, as illustrated in FIG. 19D, using the master stamper, and following a well-known replication process, read only optical recording media 104 are mass-produced. In FIG. 19D, the reference number 105 indicates a pit.
In the above fabrication process of the related art, when adjusting the pit depth, it is necessary to control the exposure of the glass substrate 101, and to form pits having different depths. In this case, however, the bottom is not flat, but is round or is roughened. In addition, because the pit depth is sensitive to the exposure, empirically, it is known that precise control of the pit depth is difficult. Theoretically, the pit depth can be modulated by changing the exposure, but because of the above reasons, in the related art, when fabricating a CD-type read only optical recording medium or a DVD-type read only optical recording medium, exposure is continued until the glass substrate 101 is exposed, and signals corresponding to pit length modulation are recorded in the CD-type or DVD-type read only optical recording medium of the related art.